This invention relates to automatic impedance tuners used in load pull testing of power transistors and noise testing of low noise transistors; said tuners allow synthesizing appropriate RF reflection factors (or complex impedances) at the input and output of said transistors [1].
A popular method for testing and characterizing RF components (transistors) for high power or low noise operation is “load pull” and “source pull”. Load pull or source pull are automated measurement techniques used to measure Gain, Power, Efficiency and other characteristics of the DUT, employing source and load impedance tuners and other test equipment, such as signal sources, directional couplers, test fixtures to house the DUT (device under test, typically an RF transistor) and input and output power meters (FIG. 1) [2]. To measure noise figure and noise parameters of a low noise device a similar setup is used (FIG. 2), in which the signal source is replaced by a calibrated standard noise source and the power meter by a sensitive noise figure analyzer, following a signal amplifying, low-noise amplifier [3]. The tuners in particular are used in both setups, in order to manipulate the RF impedance conditions under which the DUT is tested.
A popular family of electro-mechanical (automated) tuners are the “slide-screw tuners” shown schematically in FIG. 3. Slide screw tuners comprise a slotted low loss airline (slabline) with a test port adjacent to the DUT, one or more remotely controlled mobile carriages, which can travel parallel to the axis of said slabline, and carry one or more RF probes each [4]. FIG. 4 shows schematically a perspective cross section of a slabline (2) and a tuner probe. Said probes (1) are insertable (4) into and can slide inside the slot of the slabline. The probes can also move horizontally (5) parallel to the center conductor (3). Typically all probes (1, 9, 14, 20, 28, 36, 45) disclosed in this invention and shown in FIGS. 4, 7, 8, 9, 10, 11, 12, correspondingly, are made of conductive material and are attached to vertical axes (52, 53, 54, 22, 33, 41, 51) [4, 5, 6]. Said vertical axes are driven by stepper motors and allow very accurate positioning control of the distance between the probes and the center conductor of the slablines [S] in FIG. 4. Wide probes are used for low frequencies, narrow probes for high frequencies. The probes have a concave bottom so that they create high capacitive coupling with the center conductor and insert a conductive body between the center conductor and the sidewall of the slabline, where most of the electric field is concentrated.